Optical current transformer

ABSTRACT

An optical current transformer in which two light beams coming from the same source successively pass through: (1) a first polarizer, a first material having a magnetic rotational power influenced by the electrical current to be measured and a first analyzer, to end up on a photoelectric detector; and (2) a second polarizer, a second material having a magnetic rotational power influenced by a measuring electrical current and a second analyzer, to end up on the same photoelectric detector. The measuring current is dependent on the detector for maintaining a constant flux on the latter. The angle between the polarization planes of the polarizers and the analyzers of each pair is 45*.

United States Patent 1 1 Lesueur [54] OPTICAL CURRENT TRANSFORMER [75]Inventor: Gilbert Lesueur, Aix-Les-Bains,

France [73] Assignee: Alsthom-Savoisienne, Saint-Owen,

France [221 Filed: Oct. 22, 1971 21 Appl.No.:191,664

[30] Foreign Application Priority Data 11 1 3,708,747 Jan.2,1973

Bensel ..324/96 Pelenc ..324/96 [57] ABSTRACT An optical currenttransformer in which two light beams coming from the same sourcesuccessively pass through: (i) a first polarizer, a first materialhaving a magnetic rotational power influenced by the electrical currentto be measured and a first analyzer, to end up on a photoelectricdetector; and (2) a second polarizer, a second material having amagnetic rotational power influenced by a measuring electrical currentand a second analyzer, to end up on the same photoelectric detector. Themeasuring current is dependent on the detector for maintaining aconstant flux on the latter. The angle between the polarization planesof the polarizers and the analyzers of each pair is 45.

4 Claims, 1 Drawing Figure l OPTICAL CURRENT TRANSFORMER BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates to adevice for measuring the magnitude of an electrical current, using theFaraday effect.

2..Description of the Prior Art In known devices of this kind, apolarized light beam traverses a material, such as flint-glass, with amagnetic rotating power subject to a magnetic field produced by of lightpolarization angle into a variation in the intensi-' ty of the lightflux. The latter is detected .by a photoelectric detector.

It has already been proposed, in an effort to increase precision, toemploy a measurement method by means of compensation which consists ofcanceling out the rotation of the polarization plane of the light beamby means of a rotation in the opposite direction through the same angle.This rotation in the opposite direction is obtained by interspersing, inthe path of the light beam and upstream from the analyzer, a materialwith a magnetic rotating power and suitably placed in a magnetic fieldproduced by a current which is the measuring current.

This compensation method involves numerous inconveniences. It increasesthe losses in light transmission; in effect, the light beam mustsuccessively go through two materials with magnetic rotating power,instead of just one. On the other hand, the beam must be kept polarizedbetween the two materials with the magnetic rotating power; in the casewhere the material serving for the measurement and the material servingfor compensation are far removed from each other (for example, forreasons of electrical insulation), this setup requires expensive lightbeam guidance devices. Above all, the electrical energy used forcompensation is rather considerable, because this method always requiresan equal number of compensation ampereturns.

SUMMARY OF THE INVENTION The object of the invention is to provide anoptical current transformer which is as precise as transformers usingthe compensation method mentioned above and which considerably reducesthe electric power con sumption, while permitting light to be guided byoptical fibers, because the light transmitted through a certain distanceis not polarized.

The optical current transformer, according to the invention, isremarkable by virtue of the fact that it involves two light beams, thefirst of which successively passes through the following: a firstpolarizer, a first material with magnetic rotating power and placed in amagnetic field produced by a current to be measured, a first analyzer,and then impinging upon a photoelectric detector; the second one ofthese light beamspasses successively through the following: a secondpolarizer,

a second material with magnetic rotating power and placed in a magneticfield produced by a current which is the measuring current, a secondanalyzer, and then impinging upon the same photoelectric detector.Electronic means adjust the measuring current as a function of the lightflux received by the photoelectric detector, so as to keep this flux ata constant value equal to one half of the flux which would be receivedby the photoelectric detector if the polarization plane of the first orsecond light beam were merged, respectively, upon leaving the first orsecond material with magnetic rotating power, with the polarizationplane of the first or second analyzer. The angle between thepolarization planes of the polarizer and the analyzer is for each of thetwo lightbeams.

Later on we will see that the simultaneous implementation of theseconditions enables us to obtain a simple relation: equality orproportionality between the current to be measured and the measuringcurrent.

BRIEF DESCRIPTION OF THE DRAWING The single figure is a schematicdiagram illustrating one way of implementing the invention, given hereby way of example, without any restrictions.

DESCRIPTION OF THE PREFERRED EMBODIMENT Two light beams 10 and 20 aretransmitted by one and the same light source 30, but they could also betransmitted by different sources. The light beam 10 successively passesthrough the following: a polarizer 11, a body 12 with a magneticrotating power, and an analyzer 13. It is then directed toward aphotoelectric detector 40. The light beam 20 successively passes throughthe following: a polarizer 21, a body 22 with magnetic rotating power,and an analyzer 23. It is then directed, like beam 10, to thephotoelectric detector 40. The body 12 is subjected to the magneticfield of a winding 14 through which the current I to be measured runs.The body 22 is subjected to the magnetic field of a winding 24 throughwhich runs a measuring current I which is regulated by an electronicservo device energized by the electrical signal transmitted by detector40.

In one preferred version, the bodies 12 and 22 are, for example, denseflint-glasses and have the shape of a cylinder. Each one of them isarranged so as to be traversed along its axis by one of the light beams10 and 20. In order to reduce the lengths of the paths where the lightbeams must remain polarized, the polarizers and the scanners can bearranged against the faces of bodies 12 and 22. In the portions of theirpaths included between the light source 30 and polarizers 11 and 21, andbetween analyzers l3 and 23 and the photoelectric detector 40, the lightbeams 10 and 20 can be guided by optical fibers. This property isinteresting and worthwhile when body 12, influenced by the magneticfield of the current I to be measured, is separated from the lightsource 30 and from the photoelectric detector 40, for reasons ofelectrical insulation.

The elements in the upper part of the figure: references 11, 12, 13, 14,are arranged, for example, at the top of a support insulator, and theother elements of the figure are at the bottom of this insulatorsupport.

The polarization plane of polarizer 11 makes an angle (11, of: 45 withthat of the analyzer 13; likewise, the polarization plane of polarizer21 makes an angle d), of: 45 with that of the analyzer 23.

Photoelectric detector 40 transmits an electrical signal which is afunction of the intensity of the received light flux J. This light fluxis the sum of flux J,

due to the light beam 10 and the flux 1, due to the light beam 20.

Let us assume that A, is the light intensity of beam 10 at the output ofpolarizer 11, A, that of the beam 20 at the output of polarizer 21, 6,the angle of rotation which the polarization plane of beam 10 undergoesupon traversing body 12, and 6 the angle of rotation which thepolarization plane of beam 20 undergoes upon traversing body 22. If weneglect the transmission losses, the light fluxes J, and J, are equalto:

J, =A, cos a l J =A cos a 2,

where a, is the angle which the polarization plane of the light beam 10at the output of body 12 makes with the polarization plane of analyzerl3, and a 2 is the angle which the polarization plane of the light beam20 at the output of body 22 makes with the polarization plane of theanalyzer 23.

1 +1 A, cos 0,) A cos (qb 9 After trigonometric transformation, we getthe followmg:

V101, A /z [14, cos 2 (d), 0,) A} cos 2 100 2 2)] The expression forlight flux J received by photoelectric detector 40 can thus be brokendown into a first constant term and a second term that is a function ofthe angles 0, and

Electronic servo device 50 regulates the measuring current magnitude Iso as to obtain a rotation of the polarization plane of beam 20 in body22, so that the second term above will be zero. Consequently, when thereis equilibrium, the flux received by photoelectric detector 40 will beequal to k (A, A that is to say, half of the light flux whichphotosensitive element 40 would receive if the polarization planes ofbeams and 20, at the outputs of the first and second bodies withmagnetic rotating power, were merged with the polarization plans,respectively, of analyzers l3 and 23.

According to one possible version of the electronic servo device, thelatter is made up of an isolating amplifier 51 whose input terminal isconnected to the photoelectric detector 40, a subtractor 52 with twoinputs, one of which is connected to the output of the isolatingamplifier 51 while the other one is connected to an amplifier 53 whichprovides a signal corresponding to the constant term 1% (/f, A in theprocess suitably amplifying the signal furnished by a photoelectricdetector 54, receiving a light flux proportional to this constant term,that is to say, a light flux taken either from beam 10 or beam or fromthese two beams with the help of semireflecting plates or opticalfibers, or directly from source by means of a different light path, asshown at 55 in the figure, and an amplifier 56 which is controlled bythe difference signal transmitted by subtractor 5 2 and which furnishesthe power current for winding 24.

The calculation shows that the disequilibrium term of the signaltransmitted by the photoelectric detector is the product of the phaseshift multiplied by a coefficient, which product very slowly decreaseswith 6 the sensitivity of the system is thus practically constant.

In a first version, we give the same intensities A, and A to the lightbeams 10 and 20 at the output of the polarizers 11 and 21. Theequilibrium of the servo device 50 is then obtained when 6, and 6 areequal in terms of absolute value: they are equal in terms of relativevalue if d), and da, are in opposite directions, and they have oppositerelative values if d), and d), are in the same direction. In this case,one can proceed. to a direct reading because the magnitude of themeasuring current I is equal to that of the current I to be measured.

According to another version, we give angles 0, and 9 sufficiently smallabsolute values, that is, less than 5, so that one can equate the sineto the arc, and the equilibrium condition of the servo device 50 is thenwritten as follows:

It is no longer necessary, as before, for angles 8, and 0-, to be equal.In selecting A larger than A,, the angle 6 will be smaller than theangle 6,, which causes the dissipation of the electrical energy in thewinding 24 to be smaller than that of the measurement winding 14. Themagnitude of the current I to be measured is then proportional to thatof the measuring current I,,.

It is noted that when angles :1), and (I), are in the same direction,the direction of the measuring current I is such that the rotation ofbeam 20 in body 22 and the rotation of beam 10 in body 12 will be inopposite directions. In the case where 4), and d), are in oppositedirections, the direction of the measuring current I is such that therotation of beam 20 in body 22 and the rotation of beam 10 in body 12will be in the same direction.

The optical current transformer, which is the object of this invention,can be applied particularly advantageously in measuring the current of ahigh or very high voltage line.

I claim:

1. An optical current transformer comprising:

a. a first light path comprising in sequence:

a first polarizer,

a first magneto-optical element subject to a magnetic field produced bya current to be measured,

a first analyzer whose polarization plane forms an angle of 45 with thepolarization plane of said first polarizer,

a photoelectric detector for generating a control signal dependent uponthe intensity of light impinging thereon;

a second light path comprising in sequence:

a second polarizer,

a second magneto-optical element subject to a magnetic field produced bya measuring current,

a second analyzer whose polarization plane forms an angle of 45 withsaid second polarizer, and

said photoelectric detector;

0. means for generating first and second light beams along said firstand second paths, respectively, so that said beams impinge upon saidphotoelectric detector;

d. electronic means responsive to said control current for controllingsaid measuring current as a function of the intensity of light impingingupon said photodetector to maintain the intensity at a constant valueequal to one half of the intensity which would impinge upon saidphotodetector if the polarization planes of said first and second lightbeams at the outputs of said first and second magneto-optical elementswere merged, respectively, with the polarization planes of said firstand second analyzers.

2. An optical current transformer according to claim 1 wherein saidfirst and second light beams have the same intensity at the outputs ofsaid first and second polarizers.

3. An optical current transformer according to claim 1 wherein theplanes of polarization of said first and second light beams are rotatedthrough an angle of less than 5 by said first and second polarizers,respectively, and wherein the intensity of said first light beam at theoutput of said second polarizer.

4. An optical current transformer according to claim 1 wherein saidelectronic means comprises subtrac'tor circuit means responsive to saidcontrol signal and to a signal representative of said constant value forcontrolling said measuring current.

1. An optical current transformer comprising: a. a first light pathcomprising in sequence: a first polarizer, a first magneto-opticalelement subject to a magnetic field produced by a current to bemeasured, a first analyzer whose polarization plane forms an angle of45* with the polarization plane of said first polarizer, a photoelectricdetector for generating a control signal dependent upon the intensity oflight impinging thereon; b. a second light path comprising in sequence:a second polarizer, a second magneto-optical element subject to amagnetic field produced by a measuring current, a second analyzer whosepolarization plane forms an angle of 45* with said second polarizer, andsaid photoelectric detector; c. means for generating first and secondlight beams along said first and second paths, respectively, so thatsaid beams impinge upon said photoelectric detector; d. electronic meansresponsive to said control current for controlling said measuringcurrent as a function of the intensity of light impinging upon saidphotodetector to maintain the intensity at a constant value equal to onehalf of the intensity which would impinge upon said photodetector if thepolarization planes of said first and second light beams at the outputsof said first and second magneto-optical elements were merged,respectively, with the polarization planes of said first and secondanalyzers.
 2. An optical current transformer according to claim 1wherein said first and second light beams have the same intensity at theoutputs of said first and second polarizers.
 3. An optical currenttransformer according to claim 1 wherein the planes of polarization ofsaid first and second light beams are rotated through an angle of lessthan 5* by said first and second polarizers, respectively, and whereinthe intensity of said first light beam at the output of said secondpolarizer.
 4. An optical current transformer according to clAim 1wherein said electronic means comprises subtractor circuit meansresponsive to said control signal and to a signal representative of saidconstant value for controlling said measuring current.